This invention relates to a distributed restoration algorithm (DRA) network, and more particularly to a method for isolating the location of a fault in the network and the apparatus for effecting the method.
In a telecommunications network, certain portions of it may be provisioned with the ability to restore traffic that has been disrupted due to a fault or a malfunction at a given location of the network. Such portion(s) of the network that has been provisioned with the restoration ability is known as a distributed restoration algorithm (DRA) network, or domain. The nodes, or switches, in the DRA domain each are equipped with an algorithm and associated hardware that allow the nodes each to begin to look for a new path for restoring the disrupted traffic once that node senses a disruption. Each of the nodes is interconnected, by means of at least one link or span, to at least one other node. In other words, the plurality of nodes in a network are interconnected to other nodes by a plurality of links. In addition to routing traffic, the links also provide to each node signals that inform the node of the operational status of the network. Thus, a signal is provided to each node to inform the node that traffic is being routed among the nodes effectively, or that there has been a malfunction somewhere in the network and that an alternate route or routes are required to reroute the disrupted traffic.
Conventionally, when everything is operating correctly, an idle signal, or some other similar signal, is propagated among the various nodes of the network to inform those nodes that traffic is being routed correctly. However, if a fault occurs somewhere in the network that disrupts the flow of traffic, an alarm is sent out from the fault location and propagated to the nodes of the network. Such alarm signal causes the equipment in the network downstream of the fault location to go into alarm. To suppress the alarm in the downstream equipment, a follow-up signal is sent.
This prior art method of sending out an alarm signal from the location of the fault aids in the fault isolation along a single link. Unfortunately, the standard that requires the sending of an alarm signal downstream from the fault also requires that the downstream nodes, upon receipt of the alarm signal, further propagate it downstream. As a consequence, since all nodes in the network will receive the alarm signal within a short period of time after the fault has occurred, it becomes very difficult, if not downright impossible, for the management of the network to identify the custodial nodes of a failed link, or the site where the fault occurred. This is due to the fact that, in addition to the custodial nodes, many other nodes in the network likewise are in receipt of the alarm signal.
There is therefore required a method in which the true custodial nodes of a failed link be made aware that they indeed are the custodial nodes. Putting it differently, a method is required to differentiate the alarm signal received by nodes other than the custodial nodes from the alarm signal received by the custodial nodes, in order to preserve the accepted practice of sending an alarm signal to downstream-equipment.
Since in most instances a distributed restoration domain is a portion of an overall telecommunications network, or a number of different networks, it is therefore also required that the status of whatever signals received by the nodes outside of the distributed restoration domain be maintained as if there has not be any differentiation between the time when those signals are received by the custodial nodes and when those signals are received subsequently by the nodes outside the domain.
The present invention involves modifying the functionality of each node of the distributed restoration domain, so that, when in receipt of a failed signal (or an AIS signal that suppresses the alarm of downstream equipment), each node of the domain would cause the propagation of a distinct non-alarm signal (or non-AIS signal) that would accomplish the same thing as the original failed signal. Consequently, only the custodial nodes of a failed link, or those nodes that bracket a malfunctioned site, are in receipt of the true alarm or AIS signal.
Since adjacent nodes are connected by links, or spans, the kinds of signals that traverse among the nodes in a network have different formats, depending on the type of connection. In the case of a Digital Service 3 (DS3) facility, each of the nodes of the distributed restoration domain is provisioned with a converter, so that when in receipt of an AIS signal, the converter would convert the AIS signal into an idle signal (or a modified AIS signal), and propagate the idle signal to nodes downstream thereof. To achieve this conversion of an AIS signal, a modification of at least one of the C bits of the idle signal takes place. Indications of directly adjacent failures such as loss of signal (LOS), loss of frame (LOF) and loss of pointer (LOP) also result in the propagation of a modified idle signal to nodes downstream of where the fault occurred.
At the perimeter of the distributed restoration domain, each of the access/egress nodes that communicatively interconnects the domain to the rest of the network, or other networks, is provisioned such that any incoming modified idle signal is reconverted, or replaced, by a standard AIS signal so that the equipment outside of the distributed restoration domain continue to receive the standard compliant signal.
For those networks that are interconnected by optical fibers where the SONET Synchronous Transport Signal (STS-n) such as the STS 3 standard is used, each node of the distributed restoration domain is provisioned so that, in receipt of an incoming STS-N AIS signal, such AIS signal is replaced by a STS-N Incoming Signal Failure (ISF) signal. In the preferred embodiment, the Z5 bit of the STS-3 signal is changed. This modification serves the same purpose as the C bit modification to the DS3 signal.